Charge transporting elements have a support and a charge transport layer that charge moves across. Charge transporting elements include antistatic elements and charge generating elements. Antistatic elements have an antistatic layer which transports charge to prevent charge build up on the surface of the element.
In charge generating elements, incident light induces a charge separation across various layers of a multiple layer device. In an electrophotographic charge generating element, also referred to herein as an electrophotographic element, an electron-hole pair produced within a charge generating layer separate and move in opposite directions to develop a charge between an electrically conductive layer and an opposite surface of the element. The charge forms a pattern of electrostatic potential (also referred to as an electrostatic latent image). The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer and the toner image is then fused to a receiver. If desired, the latent image can be transferred to another surface before development or the toner image can be transferred before fusing.
The requirements of the process of generating and separating charge place severe limitations on the characteristics of the layers in which charge is generated and holes and/or electrons are transported. For example, many such layers are very soft and subject to abrasion. This places severe constraints upon the design of charge generating elements. Some configurations cannot provide a reasonable length of service unless an abrasion resistant overcoat layer is provided over the other layers of the element. This presents its own problems, since charge must be able to pass through the overcoat.
The resistivity of an overcoat has major consequences in an electrophotographic system. If the overcoat has high resistivity, the time constant for voltage decay will be excessively long relative to the processing time for the electrophotographic element and the overcoat will retain a residual potential after photodischarge of the underlying photoreceptor. The magnitude of the residual potential depends upon the initial potential, the dielectric constants of the various layers, and the thicknesses of each layer. A solution has been to reduce the thickness of the overcoat layer. Another solution is to provide an overcoat that is conductive. The overcoat must, however, not be too conductive. The electrophotographic element must be sufficiently electrically insulating in the dark that the element neither discharges excessively nor allows an excessive migration of charge along the surface of the element. An excessive discharge ("dark decay") would prevent the formation and development of the electrostatic latent image. Excessive migration causes a loss of resolution of the electrostatic image and the subsequent developed image. This loss of resolution is referred to as "lateral image spread". The extent of image degradation will depend upon processing time for the electrophotographic element and the thicknesses and dielectric constants of the layers. It is thus desirable to provide an overcoat that is neither too insulating nor too conductive.
The triboelectric properties of the overcoat must be matched to the triboelectric properties of the electrophotographic toner used to develop the electrostatic latent image. If the triboelectric properties are not matched, the electrophotographic element will triboelectrically charge against the electrophotographic toner. This causes disruption of the charge pattern of the electrostatic latent image and results in background in the resulting toner image. For example, an overcoat can triboelectrically match a particular negatively charging toner, but not triboelectrically match another toner that charges positively.
Silsesquioxanes are siloxane polymers, sometimes represented by the formula (RSiO.sub.1.5).sub.x, that are commonly prepared by the hydrolysis and condensation of trialkoxysilanes. U.S. Pat. No. 4,027,073 to Clark teaches the use of silsesquioxanes as abrasion resistant coatings on organic polymers. Typical applications include scratch resistant coatings on acrylic lenses and transparent glazing materials. This patent teaches that a preferred thickness for good scratch resistance is from 2 to 10 micrometers. U.S. Pat. No. 4,439,509 to Schank teaches photoconducing elements for electrophotography that have silsesquioxane coatings. The silsesquioxane overcoats have a thickness of from 0.5 to 2.0 micrometers. The patent indicates that this thickness optimizes electrical, transfer, cleaning and scratch resistance properties. This contrasts with U.S. Pat. No. 4,027,073, which teaches that a preferred thickness of a silsesquioxane layer, for good scratch resistance, is from 2 to 10 micrometers. U.S. Pat. No. 4,923,775 to Shank teaches that methylsilsesquioxane is preferred since it produces the hardest material in comparison to other alkylsilanes.
U.S. Pat. No. 4,595,602 to Schank teaches a conductive overcoat of cross-linked "siloxanol-colloidal silica hybrid" having a preferred thickness of from 0.3 to 5.0 micrometers. Cross-linkable siloxanol-colloidal silica hybrid was reacted with hydrolyzed ammonium salt of an alkoxy silane. The patent states:
"the ionic moiety of the ammonium salt of an alkoxy silane is both uniformly distributed throughout the overcoating and permanently anchored in place thereby providing sufficient and stable electrical conductivity characteristics to the overcoating under a wide range of temperature and humidity conditions." (col. 6, lines 45-51) PA1 "By reacting these ammonium salts of alkoxy silanes with a cross-linkable siloxanol-colloidal silica hybrid material, the moisture sensitivity of the resulting films can be modified so that satisfactory control of the electrical properties of these overcoats can be achieved over an extended relative humidity range of about 10 percent to about 90 percent. Moreover, the overcoatings of this invention permit thicker protective coatings to be used thereby extending the useful life of the photoreceptor. It is hypothesized that when migratable ionic components such as conventional stabilizing acids and alkali metal catalysts are present in a cured cross-linked siloxanol-colloidal silica hybrid material overcoating, the photoreceptor may initially perform well under ordinary ambient conditions. However, upon extended xerographic cycling even under ordinary ambient conditions, repeated exposure to the applied electric field causes the migratable ionic components to migrate to the interface between the overcoating and the photoreceptor thereby forming a concentrated region or layer of ionic components which becomes progressively more electrically conductive. This electrically conductive interface region is believed to be the principal cause of print deletion, particularly at elevated temperatures and high humidity." (col. 6, lines 18-43)
The patent contrasts this with a overcoat layer having migratable ionic species:
Solid electrolytes, also referred to as solid ionic conductors, are solid materials in which electrical conductivity is provided by the motion of ions not electrons. A variety of solid electrolytes are inorganic crystals. Others are complexes of an organic polymer and a salt, such as complexes of poly(ethylene oxide) and alkali metal salt. "Electrolytes Dissolved in Polymers", J. M. G. Cowrie et al, Annu. Rev. Phys. Chem., Vol. 40, (1989) pp. 85-113 teaches various solid electrolytes. "Solid Ionic Conductors", D. F. Shriver et al, Chemical and Engineering News, Vol. 63, (1985) pp. 42-57; teaches a number of solid electrolytes including a salt-polyphosphazene complex. "Polymer Electrolytes", J. S. Tonge et al, Chapter 5, Polymers for Electronic Applications, ed. J. H. Lai, CRC Press, Boca Raton, Fla., 1989, pp. 157-210, at 162; teaches solid electrolytes having highly flexible, low T.sub.g siloxane backbones. "Fast Ion Conduction in Comb Shaped Polymers", J. M. G. Cowrie, Integration of Fundamental Polymer Science and Technology, Vol. 2, Elsevior Publ., New York, (1988), pp. 54-62; also teaches a solid electrolyte having a siloxane backbone. Electrical surface conductivities for polymeric and inorganic solid ion conductors are in the range of about 1.times.10.sup.-8 to 10 (ohms/sq).sup.-1. (Surface conductivity is equal to conductivity divided by thickness and is expressed as (ohms/square).sup.-1. Surface resistivity is equal to resistivity divided by thickness and is expressed as ohms/square. For example, a resistivity of 1.times.10.sup.14 ohms-cm, for a layer having a thickness of 5 microns, equates to a surface resistivity of 2.times.10.sup.17.) Solid electrolytes are used for applications including rechargeable lithium batteries, electrochemical sensors, and display devices. Polymeric solid electrolytes tend to be soft materials with little mechanical integrity.
A problem seen in siloxane and silane coatings is a tendency to crack with stress and aging. U.S. Pat. No. 4,227,287 to Frye teaches silicone polycondensates including polysiloxane polyether copolymers having a general structure that can be written: ##STR1## The patent teaches that the addition of about 4 weight percent of these copolymers to the total solids for a polysiloxane produces an aesthetically better coating that is less subject to stress cracking.
It is therefore desirable to provide antistatic elements, glassy solid electrolytes, and charge generating elements which provide both good resistance to abrasion and useful charge transport properties.